It boils down to who gets a grape

In a recent study, brown capuchin monkeys trained to exchange a granite token for a cucumber treat often refused the swap if they saw another monkey get a better payoff - a grape.

Instead, they often threw the token, refused to eat the piece of cucumber or gave it to the other capuchin after viewing the lopsided deal, said Emory University researcher Sarah Brosnan.

She said the results indicate man and monkey might have inherited a sense of fairness from an evolutionary ancestor.

"This implies we evolved this way," said Brosnan, whose work with colleague Frans B.M. de Waal is reported in today's issue of the journal Nature.

The trait might have helped species cooperate and survive, Brosnan said.

However, Charles Janson, who studies capuchins at the State University of New York at Stony Brook and was not involved in the research, suggested the monkeys' behavior might have been learned in captivity rather than inherited.

Brosnan said she doubted the behavior was learned, saying most animals "cannot learn things which they do not naturally do in the wild."

"More importantly, however, learning behavior requires that individuals get rewarded for performing a specific behavior," Brosnan said. "In our test, the subject actually received less reward for refusing to exchange."

The researchers, at Emory's Yerkes National Primate Research Center, studied five female monkeys, testing them two at a time.

When both monkeys were given a cucumber slice after handing over the token, they completed the trade 95 percent of the time.

But when one was given the tastier grape for the same amount of work, the rate of cooperation from the other monkey fell to 60 percent. And when one didn't have to do anything to get a grape, the other made the trade for the cucumber 20 percent of the time.

The scientists concluded that capuchins apparently measure rewards in relative terms, comparing their rewards to those available and their efforts to those of others.